1. Field of the Invention
Embodiments of the invention relate to a display device, and more particularly to a liquid crystal display device. Although embodiments of the invention are suitable for a wide scope of applications, it is particularly suitable for an inverter circuit that supplies power to a backlight of a liquid crystal display device.
2. Description of the Related Art
As the information society develops, the need for display devices in various shapes increases. Various types of flat panel display devices have been developed, such as LCD (liquid crystal display), PDP (plasma display panel), ELD (electro luminescent display) and VFD (vacuum fluorescent display). The liquid crystal display device is the flat panel display device that has been more widely used than any other types of display devices because the liquid crystal display device has the combined characteristics of low power consumption and high picture quality.
Generally, the liquid crystal display device controls light transmittance of a layer of liquid crystal molecules by using an electric field to display a picture. To this end, the liquid crystal display device includes a liquid crystal display module, a drive circuit for driving the liquid crystal display module, and a case. The liquid crystal display module includes a liquid crystal display panel and a backlight assembly, which irradiates light onto the liquid crystal display panel. The liquid crystal display panel and the backlight assembly are combined together into a liquid crystal display module so as to prevent light loss.
The liquid crystal display panel includes two substrates which face each other, and a liquid crystal interposed between the two substrates. One of the two substrates has thin film transistors (hereinafter, referred to as ‘TFT’) and is referred to as an array substrate. The other of the two substrates has color filters and is referred to as a color filter substrate. The liquid crystal display panel is a non-emissive type of display panel that uses light from the backlight assembly to display a picture.
A backlight assembly can either be an edge type or a direct type depending on the location of the light sources within the backlight. In the edge type, the light source of the backlight is located at the side surface of the liquid crystal display panel and the light is supplied through a waveguide. In the direct type, a plurality of light sources is located directly under the liquid crystal display panel.
Recently, the liquid crystal display device has been enlarged and is now more commonly used in televisions. The direct type backlight is mainly used for a liquid crystal display device in a television because high brightness can be obtained. The light source used in the backlight can be a cold cathode fluorescent lamp (CCFL) or an external electrode fluorescent lamp (EEFL).
FIG. 1 is a cross-sectional diagram representing a direct type backlight assembly of the related art, and FIG. 2 is an enlarged diagram of part ‘A’ in FIG. 1. Referring to FIGS. 1 and 2, the direct type backlight assembly includes lamps 10, a cover bottom 14 that houses the lamps 10, a reflection sheet 12 installed inside the cover bottom 14, and an inverter printed circuit board (hereinafter, referred to as ‘PCB’) 16 disposed outside of the cover bottom 14. A diffusion plate is positioned over the cover bottom 14 that houses the lamps 10, and a plurality of optical sheets are deposited on the diffusion plate.
The cover bottom 14 includes a bottom surface and side surfaces. The reflection sheet 12 is adhered to the inside of the bottom surface and side surfaces of the bottom cover 14 with double faced tape. The reflection sheet 12 reflects the light incident from the lamps 10 to the front of the backlight assembly.
The lamps 10 are external electrode fluorescent lamps (EEFL) that are arranged in parallel. An external electrode fluorescent lamp has a glass tube in which phosphorus is spread over the inner wall of the glass tube. Then, an inert gas is injected into the glass tube and external electrodes are installed at both ends of the glass tube.
The inverter PCB 16 is disposed on the outside bottom surface of the cover bottom 14. A transformer 55 for driving the lamps 10 is mounted on the inverter PCB 16, as shown in FIG. 2. An inverter circuit (not shown) is also on the inverter PCB 16. The inverter circuit includes a switching part (not shown) that receives DC power from an external power supply for conversion into an AC signal, a transformer 55 that boosts the AC signal generated from the switching part to supply a boosted AC signal to the lamps 10, and a controlling part that detects current supplied to the lamps 10 so as to control the switching part.
The lamps 10 can be driven by an inverter circuit using an individual drive method in which lamps correspond to transformers in a one-to-one relation. In the alternative, the lamps 10 can be driven in a parallel method in which a plurality of lamps correspond to one transformer in a many-to-one relation. In the case of the individual drive method, one transformer is used for driving one lamp, so that a low-current output transformer can be used to drive the single lamp. In the case of the parallel drive method, many lamps are driven by one transformer so that a high-current output transformer is used to provide power to a plurality of lamps.
FIG. 3 is a diagram showing a parallel drive method in which a plurality of lamps are driven using a single high-current output transformer. External electrode fluorescent lamps can be driven by the parallel drive method, as shown in FIG. 3. More specifically, a plurality of external electrode fluorescent lamps 10 can be commonly connected to a common electrode 40 to receive a high voltage AC from the inverter circuit 50.
As shown in FIG. 2, the inverter circuit 50 includes the high-current output transformer 55 mounted on the inverter PCB 16. The inverter PCB 16 includes an epoxy glass fiber substrate 16C, and first and second copper thin film layers 16A and 16B, which are coated onto the upper and lower surfaces of the glass fiber substrate 16C. The copper thin films are not located on areas of the upper and lower surfaces of the inverter PCB 16 that correspond to an area where the transformer 55 is mounted on the inverter PCB 16. In the case of the upper surface of the inverter PCB 16 closest to the high output of the transformer 55, an area of the copper thin film 16a is omitted to prevent the risk of discharge due to a high voltage output from the high voltage output of the transformer 55 to the copper film 16a. On the other hand, in the case of the lower surface of the inverter PCB 16 adjacent to the high output of the transformer 55, an area of the copper thin film 16b is omitted in consideration of an insulating internal pressure of the inverter PCB 16.
In the liquid crystal display device of the related art, as described above, the copper thin film 16b is omitted at the lower surface of the inverter PCB 16 which corresponds to a mounting area of the high output transformer 55 so that magnetic flux generated from the high output transformer 55 impact the outside surface of the cover bottom 14. The number of magnetic fluxes impacting thereto, i.e., the number of leaked magnetic fluxes, increases as the output voltage of the high output transformer 55 increases. As a result of the impacting magnetic fluxes, eddy current flows along the outside surface of the cover bottom 14. The eddy currents cause a localized heating in the cover bottom 14. The localized heating of the cover bottom 14 heats up the high output transformer 55 of the liquid crystal display device of the related art so as to cause the high output of the transformer 55 to operate inefficiently and waste power.